The capacity of rotary compressors of various types is usually controlled by an axially or tangentially displaceable sliding valve opening one or more counterflow channels between the operating chamber and the inlet side of the compressor. Such an arrangement is shown in FIG. 1 which shows a section through a screw compressor of the SRM type, also known as a Lysholm or twin-screw compressor. In principle, the same control system is used in the Globoid compressor.
The rotary compressor in FIG. 1 comprises an operating chamber 1 with rotors, an axially displaceable slide 2 which also forms an outlet gate 6, the inlet side 3 and outlet side 4 of the compressor, and a counterflow gate 5. The counterflow gate communicates with the inlet side of the compressor and its size is determined by the displacement of the slide 2.
Rotary compressors of the type mentioned are displacement compressors with no operating valves in the inlet or outlet gates. For optimal efficiency an internal compression occurs in the operating chamber when the inlet has been closed and before the outlet is opened. FIGS. 2a, b and c show the axial built-in volume variation and the position of the sliding valve when the counterflow channel is closed and when it is displaced towards the outlet plane. The following equation is applicable for optimal efficiency: ##EQU1## see FIG. 3.
P2 is the outlet pressure, P1 is the inlet pressure, V1 is the maximum volume, V2 is the operating volume immediately before the outlet gate is opened and n is the polytrophic exponent.
The above designations are also applicable to the full-load case shown in FIG. 2b, that is when the counterflow channel is completely closed. At a drop in the capacity requirement, the sliding valve is displaced towards the outlet plane, FIG. 2c, and the situation aimed at for optimal operation corresponds to ##EQU2##
It can be seen from FIG. 2c that when the sliding valve is displaced towards the outlet plane the discharge gate will become smaller, which in this case results in the volume V4&lt;V2 and that the volume withdrawn V3&lt;V1. In this manner the above optimum ratio between pressure and volume may be maintained.
U.S. Pat. No. 3,088,658 (Wagenius) describes a rotary compressor with a rotating valve for control of capacity and volume ratio. This comprsessor is of a complex mechanical design with valve housings on one or both sides of the operating chamber and with complex curved shapes of the rotating valve or valves, requiring accurate and expensive machining operations. The rotating valves will require complicated closed loop control systems for their accurate positioning. Further, any adjustment of the compressor capacity will involve accurate positioning of mechanical masses and will therefore be slow unless complicated, powerful and expensive control systems are provided. Also the location and size of the various ports in the wall of the operating chamber is restricted by the necessity of the ports communicating with the valve housing.
U.S. Pat. No. 4,042,310 (Schibbye) discloses a rotary compressor with a sliding or rotary valve for capacity control. This compressor has the same disadvantages as the compressor described in the above-mentioned Wagenius patent.
Japanese Patent Publication 59-131 791 (Toyoda Jido Shokki Seisakusho K.K.) discloses a rotary compressor with lift valves for capacity control.
In a compressor with capacity control by means of counterflow gates, the built-in volume ratio will vary when the capacity is varied, resulting in a non-optimal pressure ratio and consequent power losses. These losses are particularly high in a compressor for use in a refrigeration system or in a heat pump system due to the high density of the fluid media used in such plants.